Repeater systems can serve a wide frequency band, for example a complete 3GPP band. A frequency band typically contains multiple carriers carrying information according to different communication technologies and standards such as GSM, UMTS, LTE or the like.
A repeater system of this kind may for example be used on a train to provide network coverage within the train.
High-speed train applications in this regard can present challenging RF environments. The complexities of different terrain combined with rapidly changing outdoor signal levels of the various networks can make it difficult for operators to provide the coverage and service their customers demand while travelling from city to city or to another country. To improve the reliability of wireless signals on trains, repeater systems in the shape of so-called distributed antenna systems (DAS) have proven to be cost-effective. A DAS may for example be installed within a train and serve to amplify a signal to compensate an attenuation caused by the train (due to, for example, metalized windows on high-speed trains reducing signal penetration into the train carriages, which may result in spotty coverage and dropped calls).
A DAS generally may be installed within a train and amplify, during operation, a signal between a pick-up antenna at the outside of the train and an antenna network within the carriages of the train. On the one hand, the DAS compensates the attenuation of the signals caused by the train. On the other hand, however, the signals of all users are combined and communicated via a single (or a few) common pick-up antenna mounted on the outside of the train. In case the train enters a communication cell of another base station, this can inherently cause a simultaneous handover (HO) of multiple users located inside the train and communicating via the repeater system. Thus, a large number of handovers of the multiple users located within the train may occur in a very short timeslot, which may, depending on the train speed and the overlapping area of the base stations taking part in the handover, increase the so-called handover outage probability.
One solution to alleviate this problem is to use several independent repeater systems with several pick-up antennas. For example, each carriage may be equipped with an individual repeater system. Thereby, the handover scenario of the users located on the train is split by carriage and hence into several portions as the carriages enter a handover region with overlapping cells of adjacent base stations one after the other in a time-offset manner.
For example, if a distance of 50 m is assumed between the pick-up antennas of repeater systems of adjacent carriages, the handover of users of adjacent carriages can be triggered with a time offset of 0.51 seconds, assuming the train is travelling at 350 km/h. This time difference generally may be sufficient when considering that an expected average handover time is less than 300 ms (as specified for example for a 3GPP network).
This multi-antenna concept can perform well, but has the drawback that each carriage is equipped with a separate pick-up antenna (respectively a separate DAS), which generally is costly. In addition, the delay between the handovers of the individual carriages is determined by the distance of pick-up antennas of successive carriages and the train speed. Thus, in case the handover of users in one carriage fails, there possibly is an increased interference during the handover procedures of the next carriage in that the mobile devices (UE) may try to re-connect to the (former) base station. This may increase the probability of handover outage for the following carriages.